The eternal battle to combat global warming: (thio)urea as a CO2 wet scrubbing agent

Abstract

(Thio)Urea scaffolds are best known for their importance as intermediates in organic synthesis. In this work, a mechanistic study of the reaction between urea (U), (2-hydroxyethyl)urea (U-EtOH) and thiourea (tU)/NaH in DMSO with CO₂ was carried out. While both U/tU reacted with CO₂via a 1 : 2 mechanism through the formation of the keto (thio)carbamide–carboxylate adducts (k-U/tU-CO₂⁻ Na⁺), U-EtOH gave mixed CO₂-adducts composed of organic carbonate and carbamide–carboxylate moieties (Na⁺-CO₂-U-Et-OCO₂⁻ Na⁺). Moreover, we recorded for the first time, a new type of bond, namely sodium carbamimidothiocarbonate (e-tU-SCO₂⁻ Na⁺), upon bubbling CO₂ in the DMSO solution of tU due to the persistence of the enol form (e-tU) and the better nucleophilicity of sulfur over nitrogen focal points. The reaction mechanisms were proven by 1D and 2D nuclear magnetic resonance (NMR) and ex situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopies. The stability of these bonds was studied following the changes in ¹H-NMR as a function of temperature, which indicated the reversibility of these reactions. Furthermore, the proposed mechanisms were explored theoretically via density functional theory (DFT) calculations by analyzing the energetics of the anticipated products.